An avalanche photodiode is a light detector which typically comprises a .pi.-type conductivity substrate, such as silicon, having an N-type region extending into the substrate a distance from a first major surface thereof and a P-type region extending a further distance into the substance from the N-type region, forming a P-N junction therebetween. A P.sup.+ -type contacting region extends a distance into the substrate from a second major surface thereof to provide for electrical contacting to the substrate. When a reverse bias voltage is applied to this photodiode the depletion region of the diode reaches from the P-N junction into he .pi.-type region when the peak electric field at the P-N junction is about 5-10% less than that required to cause an avalanche breakdown. A further increase in the applied voltage causes the depletion region to extend towards the P.sup.+ -type region, while the electric field throughout the diode increases relatively slowly.
Avalanche photodiodes, which are typically a few square millimeters in area, are fabricated in a silicon wafer containing potentially hundred of such diodes. In the structure described above the first manufacturing step is typically a boron ion implantation into defined regions of the wafer to form the P-type region. However, this boron ion implantation is limited by state of the art machinery such that one portion of the wafer may receive a greater areal dose than another portion. These fluctuations in areal dose may arise from non-uniformities in the ion beam pattern or from temporal variations in the ion source output. For a typical wafer thickness of about 0.18 millimeters, a diode with an implanted boron concentration about 2% higher than the average will have a breakdown voltage about 100 volts less than that of the average diode. Using presently available machinery only about half of the diodes from a wafer have a breakdown voltage in an acceptable range. Thus, it would be desirable to have a manufacturing method for avalanche photodiodes which compensated for this variation in the implanted dose so as to increase the yield of acceptable devices from a wafer.